PSI - Issue 2_B

Shohei Asako et al. / Procedia Structural Integrity 2 (2016) 3668–3675 Asako et al/ Structural Integrity Procedia 00 (2016) 000–000

3669

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1. Introduction Today, on the background of strong demand for cost efficiency, we need higher strength and toughness steels more than ever before. To meet this demand, the TMCP (Thermo Mechanical Control Process) steel which has good material properties of strength, ductility and toughness has been developed and is widely used in steel structural fields now. When we choose the structural materials for some structures, we must consider not only the strength but also the brittle fracture toughness. Although TMCP steel usually has much better brittle fracture toughness than original strength steels, we still need to improve its brittle fracture toughness so as to use it safely under various conditions. In order to enhance the fracture toughness of material without extra costs, we must clarify the micro-mechanisms of brittle cleavage fracture. The words “the micro-mechanisms” we use here is defined by Thompson and Knott (1993) as “the mechanical processes operating in materials on the scale of the microstructure”. However, the statistical models for cleavage fracture assessment and the local approaches for prediction of the macro-toughness parameters have been a prevalent trend and the micro-mechanisms of brittle fracture is not sufficiently focused on. These statistical analyses have their strong benefits in predicting material’s brittle fracture toughness without conducting actual experiments. The analysis is based on the fracture mechanics theory, in which the chemical composition and manufacturing conditions are considered as variables. We can predict the microstructure using the chemical composition and manufacturing data, on the other hand, the accuracy of prediction of the material properties is not satisfactory. It is generally thought that the controlling factors of brittle fracture toughness are grain size, strength and the existence of hard phase. However they are controlled by so various factors that it is unable to evaluate the independent influence of these each factors on brittle fracture toughness by conventional experiments like the Charpy impact test and CTOD test. There are many researches for seeking for the dependency between the micro-structure and brittle fracture toughness. Nakajima et al (1972) evaluated the brittle fracture toughness of bainite and martensite steels and indicated that their fracture transition temperature are proportional to square root of cleavage fracture facet area. For predicting the material properties accurately, it is needed to clarify the basic Martin-Meizoso et al (1994) succeeded to explain the temperature dependency of cleavage fracture stress by using these three critical steps. 1: initiation of micro-crack on hard phase 2: propagating of the microcrack along a cleavage plane of the grain faced on the hard face 3: propagation of the crack across the next grain boundary. Today, although MA is regarded as one of the main causes of deteriorating of brittle fracture toughness, we have not revealed how MA gives a bad influence on brittle fracture toughness yet and there are some theories. Mimura et al (1970) concluded that fracture toughness is deteriorated by the existence of MA because it is often observed in the trigger position of fracture. Igawa et al (1980) reported that the first microcrack was the decohesion between MA and ferrite matrix. He et al (1984) indicated that the first step of fracture is the initiation of micro-cracks on MA and then it propagated to boundaries between MA and ferrite. Chen et al (2013) investigated about the relationship between the size of MA and the shape of micro-cracks and indicated that the large MA leads brittle fracture by creating micro-cracks between mother phase and itself, on the other hand, fine MA leads to ductile fracture because voids are nucleated from such fine MA. In spite of these previous researches, the influence of micro-structures on brittle fracture toughness in bainite and its mechanism have not been fully understood. Besides, when we try to investigate the brittle fracture phenomenon in detail experimentally, we can only presume how it happened after brittle fracture came over the material because brittle fracture initiation occurs inside the structure and propagates very fast. The critical condition of the decohesion and the detailed micro deformation behavior at the boundary between MA and matrix is still unclear. Considering these lacking points just I mentioned above, we think what to do now is to observe how fracture occurs in-situ on micro structural scale. So in this research, we carried out some direct observation with specially designed specimens. We can observe directly the micro-processes of bainitic brittle fracture in-situ and the precursor phenomenon and evaluate the contribution of the factors in this system.